The overall objectives of the ongoing investigation are elucidation of fundamental biological mechanisms involved in the formation, propagation, and genetic control of antibiotic resistance plasmids and extrachromosomal elements. Modification of these processes as a result of the information obtained. OBJECTIVES SET DURING THE CURRENT YEAR: (1) Continued study of transposable genetic elements and other genetic segments on bacterial plasmids carrying antibiotic resistance determinants, (2) investigation of site-specific genetic recombination in vivo mediated by "restriction" endonucleases and other site-specific recombinational systems, (3) investigation of cryptic endonucleases detectible in vitro in cell extracts but not by in vivo phenotypic expression, (4) construction of new bacterial plasmid cloning vehicles usable in recombinant DNA experiments, and (5) investigation of expression of plasmid genes.